The present invention relates generally to electrode structures for discharge lamps.
Electrodes in short-arc discharge lamps typically operate in a high-temperature environment. Reducing the operating temperature of the electrodes is desirable in order to reduce degradation from evaporation and extend the lifetime of the lamp. The electrode operating temperature is determined by the electrical power input, which heats electrodes, and Planck's radiation law (i.e., the electro-magnetic emission of an electrode, which results in the electrode cooling). Thus, increasing the emissivity of an electrode structure will increase the heat dissipation of the electrode.
Because electrodes are routinely operated near the melting point of the electrode material (e.g., tungsten), the emissivity of an electrode structure is important parameter in discharge lamp design. For example, high-power DC lamps used in microlithography include massive anodes that are coated or microstructured to increase emissivity. Such anodes are expensive and not practical in lower-power, short-arc lamps. This technique also has the drawback that neither the coating or microstructure can be applied as close to a front portion of an electrode as desired because a non-tungsten coating will either melt or sublimate at temperatures approaching the tungsten melting point. Moreover, re-crystallization and surface diffusion will destroy tungsten microstructures over time.
Massive anodes are also not practical in some lamps because electrode size restrictions of many discharge lamps. That is, many discharge lamps are designed to accommodate only electrodes with small diameters or widths. Thus it is not always possible to reduce the electrode operating temperature at a given electrical power input by greatly increasing the size of an electrode.
FIG. 1 shows a conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil 102 is tightly wound around the electrode shaft portion 104 in one or more layers to form electrode head portion 106. Front portion 108 is condensed by over-melting the ends of coil 102. The electrode temperature is determined by the size of electrode 100, which in turn is determined by the length of coil 102, the number of coiled layers, and the diameter (or width) of the wires of coil 102.
FIG. 2 shows another conventional electrode structure for use in a ultra-high-pressure mercury lamp. Coil 202 is tightly wound around electrode head portion 204. Head portion 204, front portion 206, and shaft portion 208 are formed by shaping a conventional massive electrode material such as tungsten with conventional machining techniques such as lathing or grinding. Electrode 200 has better emissivity than electrode 100 because of the shape of front portion 206 and coil 202 is wrapped around electrode head portion 204, electrode head portion 204 being massive and can effectively conduct the heat generated in the front portion 206 to coil 202.
As noted above, however, the amount an electrode size may be increased is limited in many applications for practical and/or commercial reasons.